My revdep-rebuild did compile a few packages after the FLAC upgrade and as mentioned earlier, was happy after.

I found a report in an Arch forum on identical problems with Amarok as I experience. There seem to be a strong dependance on QT version, so I updated that and now I'm in a seemingly infinite loop of new dependency-enforced updates. As my computer is not the fastest, it has been compiling for close to 24 hours now. Every now and then, emerge stops and tells me that some library needs to be recompiled with some additional use flag. So it goes on. Looks like I'll have a complete update world when (and if) I'm done!

BlueJayofEvil, did you tried Amarok after the FLAC update? Does it run?

It eventually finished compiling (after 24 hours!) and it was still possible to reboot and everything looked fine, except that Amarok still didn't start. In a final attempt I reinstalled Amarok, and much to my surprise, it ran just fine!

I'm amazed that this dependancy hell could actually be sorted by reading the information from emerge and following it's guidance. I was on the verge of wiping the whole system clean with a fresh install when suddenly all parts fell into place. I still need to play around a bit to ensure that nothing else got broken in the process, but it looks very promising.